In conventional liquid crystal display devices, several optical sheets are generally inserted between a light guiding plate and a liquid crystal panel and usually affixed to a main frame by using double-sided adhesive tape.
However, when the liquid crystal panel is moved or forcefully struck from the outside, there have been cases where the optical sheets were damaged because they could not expand or contract freely, being firmly fixed by double-sided adhesive tape, causing crimps to occur. Likewise, there have been cases where the display surface would become contaminated when the adhesive agent of the double-sided adhesive tape melts due to heat generated by a light source, if the adhesive quality of the double-sided adhesive tape is inferior. Further, the liquid crystal display device was also preferably made thicker through the use of greater amounts of double-sided adhesive tape, thereby entailing higher costs.
Consequently, a liquid crystal display device created with the use of mechanical components or members to align and fix the optical sheets firmly in lieu of double-sided adhesive tape, has been introduced.
For example, as shown in FIGS. 6 (plane view) and 7 (sectional view at VII–VII of FIG. 6), in the liquid crystal display device 100 disclosed in Japanese Patent Laid-Open Publication No. 2002-72174, shafts 104 disposed on both sides of the floor of a main frame 102 are inserted into openings 108 provided in tabs 107 of an optical sheet 106, a guide panel 110 is installed to the main frame 102, a liquid crystal panel 112 and a case top 114 are arranged on the upper surface of the guide panel 110, and the optical sheet 106 is thereafter aligned and fixed and the liquid crystal panel 112 is installed. The shafts 104 are entirely columnar in shape, while columnar protrusions 116 having a diameter smaller than that of the lower portion of the shafts 104 are formed on top thereof, to align and fix the optical sheet 106 on both sides thereof.
According to this constitution, since the shafts 104 set on the main frame 102 fix the optical sheet 106, the probability of avoiding the adverse effect due to movement of the liquid crystal panel 112 and external vibration is increased compared to the case where the optical sheet 106 is affixed with double-sided adhesive tape.
Further, in the illumination apparatus disclosed in Japanese Patent Laid-Open Publication No. 1997-147618, to align and fix the optical sheet by using a mechanically processed frame, openings are severally provided in tabs provided on one side of a diffusion plate, a light guiding plate and a reflector, while protruding pins are formed on one side of a lower frame and holes for receiving the pins are provided on an upper frame, and the pins are inserted into the openings provided in the tabs of the diffusion plate, the light guiding plate and the reflector to hook the upper frame to the lower frame. In such manner, the diffusion plate, the light guiding plate and the reflector are fixed between the two frames.
In the known fixing method of the optical sheet described above, since adhesive tape is not used, the occurrence of crimps can be minimized to a certain extent while at the same time the contamination of the display surface due to the melting of the adhesive agent can be prevented, thereby enhancing work efficiency.
However, in the liquid crystal display device 100 disclosed in Japanese Patent Laid-Open Publication No. 2002-72174, the use of a plurality of uniquely shaped shafts 104 is required, and the shafts 104 have to be manufactured separately. Therefore, as the number of requisite components has increased, installation thereof into the main frame 102 is troublesome, thereby impairing work efficiency. In addition, the height of the protrusions of the shafts 104, which are designed to protrude from the opening of the guide panel 110, is greater than the thickness of the optical sheet 106. For this reason, it is impossible to extend the peripheral edge of the liquid crystal panel 112 to the opening portion of the guide panel 110, and the display screen of the liquid crystal panel is therefore made narrow. In addition, since the columnar protrusions 116 protrude from the upper surface of the shafts 104, it is impossible to make the liquid crystal display device 100 thinner. Moreover, since openings 108 are provided in the tabs 107 protruding at the central portion of both sides of the optical sheet 106, the optical sheet 106 must be uniquely shaped, and if openings are provided on the side edge of the optical sheet 106 without providing the tabs 107, the display screen becomes narrower due to the size of the openings.
Further, since it is fixed firmly on both sides, crimps could occur when the optical sheet expands and contracts due to heat, thereby causing damage thereto.
Furthermore, the illumination apparatus disclosed in the above-described Japanese Patent Laid-Open Publication No. 1997-147618 is aligned by fitting the pins of the lower frame into the holes of the upper frame. However, since the upper frame usually comprises a thin plate, the pins protrude from the surface of the upper frame, and to prevent this, it is necessary to make the tabs thicker. When the pins protrude from the upper frame, the liquid crystal panel cannot be placed on the pins, so the panel should be arranged in such manner as to avoid the pins, requiring the upper frame to be wider. As a result, the display screen of the liquid crystal panel becomes substantially smaller.